The present invention relates to a spread spectrum clock generator, and in particular to a spread spectrum clock generator for generating a signal whose frequency varies according to a desired profile and reducing the level of electromagnetic interference.
The demand for high-speed electronic equipment has created a number of problems for circuit designers and manufacturers. High-speed electronic equipment requires clocks that operate at relatively higher frequencies. Higher frequency operation, however, increases the level of electromagnetic interference (EMI). In addition, long traces of wire on printed circuit boards increase EMI levels. The increased level of EMI can negatively effect the operation of components in proximity to such high-speed circuitry due to the coupling of the radiated energy. Therefore, it is desirable to lower the unwanted EMI energy levels while increasing the speed of the electronic circuitry.
In the United States, the Federal Communications Commission (FCC) has established rules and standards regarding EMI emissions from electronic equipment. The EMI emission limitations are published in the Code of Federal Regulations. The FCC also enforces three standards and levies fines against violators. Compliance with the FCC standard has become quite a challenge for most circuit board manufacturers.
FIG. 1 is a spectral plot of energy versus frequency for an ideal clock. All the energy is concentrated at the clock frequency Fc. The plot also shows the FCC level. This clock exceeds that level. A well known technique to reduce the peak energy is to spread the spectrum of the clock. This technique is used, for example, in spread spectrum radio. By spreading the clock energy over a broader frequency band, no peak exceeds the FCC level, as shown in FIG. 2.
Currently, spread-spectrum clock generators are used to spread a spectrum of a clock signal. According to an embodiment of the present invention, a spread-spectrum clock generator is used to provide a finely controlled clock signal frequency thus minimizing EMI levels thereof.